1. Field of the Invention
This invention relates to novel mixtures of carboxyl group-containing polyisocyanates having an average isocyanate functionality of at least 4.1, a process for their preparation and their use as cross-linking agents in aqueous coating compositions.
2. Description of the Prior Art
The importance of aqueous lacquers and coating compositions has greatly increased in recent years due to the progressively stricter guidelines concerning the emission of solvents in the application of lacquers. Although aqueous lacquer systems have become available for many fields of application, they have in many cases only achieved the high quality of conventional, solvent-containing lacquers as regards solvent resistance and chemical resistance by a chemical cross-linking during film formation.
Prior to the present invention few attempts have been made to achieve such cross-linking of aqueous lacquers with compounds containing free isocyanate groups, i.e., to apply the concept of reactive polyurethane lacquers which is well known in the field of solvent-containing coatings to aqueous systems. DE-OS 2,708,442 describes the addition of monomeric organic diisocyanates for improving the properties of aqueous polyurethane dispersions. According to DE-OS 3,529,249, organic polyisocyanates may be used for increasing the solvent resistance and chemical resistance and for improving the wear resistance of coatings based on homo- and copolymers dispersed in water. The described positive effects on the lacquer properties may be attributed to a "urea sheath" formed around the dispersed polymer particles by the reaction of water with the isocyanate groups on the surface of the dispersed particles. Thus, the polyisocyanates used as additives do not act as cross-linking agents for aqueous dispersions of synthetic resins or synthetic resin precursors containing isocyanate reactive groups.
The preparation of an aqueous polyurethane reactive system from selected aqueous polyhydroxyl compounds which have an emulsifying action and low viscosity free polyisocyanates is disclosed in DE-OS 3,829,587. The properties of the lacquer films obtained are similar to those of known two-component lacquers containing solvent. Due to the generally poor dispersibility of organic polyisocyanates in water, however, this process is limited to special polyol dispersions. Transferring this process to other aqueous binders containing isocyanate reactive groups would require hydrophilic, water-dispersible polyisocyanate components.
Aromatic, hydrophilically modified polyisocyanates such as those described in DE-OS 2,359,613 and EP-A-61,628 are not suitable for use in aqueous lacquer systems due to their high reactivity with water and the consequent evolution of carbon dioxide. They are preferably used in the production of foams and for adhesives.
Isocyanate functional cross-linking agents for aqueous coating systems can only be prepared from less reactive (cyclo)aliphatic polyisocyanates.
Hydrophilically modified aliphatic polyisocyanates as additives for aqueous adhesives are described in EP-A 206,059. These polyisocyanates are rendered emulsifiable by the incorporation of polyether chains containing ethylene oxide units. Such hydrophilic polyisocyanates are less suitable for use in the field of lacquers due to the permanent hydrophilic character of the coating because of the relatively high polyether content.
Uretdione group- and carboxyl group-containing poly-isocyanate mixtures having average isocyanate functionalities of 1.5 to 4.0 in which the carboxyl groups are present in a storage stable form in the presence of free isocyanate groups may be prepared according to a copending application of the present Applicants (German Patent Application P 4 001 783.4 and U.S. Ser. No. 07/644,798). These carboxyl group-containing uretdione polyisocyanates can readily be stirred into water after neutralization of at least a portion of the carboxyl groups with tertiary amines. Therefore, they are suitable in principle for formulating aqueous two-component polyurethane systems.
An object of the present invention is to provide carboxyl group-containing polyisocyanates having an (average) isocyanate functionality above 4.0 which are stable in storage even though carboxyl groups and isocyanate groups are present in the same molecule; are readily soluble or dispersible in water, if necessary after at least partial neutralization of the carboxyl groups; and are suitable for use as starting components for the production of polyurethane resins and also as cross-linking agents for aqueous binders or binder components in aqueous lacquers.
This objective may be achieved with the polyisocyanate mixtures of the present invention as described hereinafter.